dotnet/wpf · error · ArgumentException

SR.ParserAttributeArgsLow with 'XmlnsDefinitionAttribute'…

Error message

SR.ParserAttributeArgsLow with 'XmlnsDefinitionAttribute' (missing XmlnsDefinitionAttribute constructor arguments)

What it means

When XmlnsCache.LoadClrnsToAssemblyNameMappingCache (invoked from AddReferencedAssemblies) parses [XmlnsDefinitionAttribute] instances, both xmlNamespace and clrNamespace must be non-empty. GetNamespacesFromDefinitionAttr returned null/empty for one of them, so an ArgumentException with ParserAttributeArgsLow is thrown — the attribute is missing required constructor arguments.

Solutions

1. Fix the [XmlnsDefinition] declaration to supply both xmlns and clrNamespace arguments
2. Check all referenced assemblies for malformed XmlnsDefinitionAttribute usages and correct or remove them
3. Rebuild the referenced assembly to rule out stale/incorrect metadata

Example fix

// before
[assembly: XmlnsDefinition("http://mycompany.com/schema")] // missing clrNamespace
// after
[assembly: XmlnsDefinition("http://mycompany.com/schema", "MyCompany.Controls")]

foreach (var attr in asm.GetCustomAttributesData())
{
    if (attr.AttributeType.Name == "XmlnsDefinitionAttribute" && attr.ConstructorArguments.Count < 2)
        throw new InvalidOperationException($"{asm}: XmlnsDefinitionAttribute missing required arguments");
}

try { AddReferencedAssemblies(references); }
catch (ArgumentException ex) when (ex.Message.Contains("ParserAttributeArgsLow"))
{ log.Error("A referenced assembly declares an XmlnsDefinitionAttribute without xmlns/clrNamespace", ex); }

When it happens

Trigger: A referenced assembly contains [assembly: XmlnsDefinition("http://ns")] with no clrNamespace, or [XmlnsDefinition(null, "Ns")], and the assembly is being scanned when references are added.

Common situations: Hand-written assembly-level attributes where the second argument was omitted; code generators emitting malformed XmlnsDefinition; conditional compilation stripping an argument; assembly references pointing at a broken/partially built library.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• ArgumentNullException(nameof(clrNamespace))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))
• ArgumentNullException(nameof(newNamespace))
• ArgumentNullException(nameof(oldNamespace))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/cb79ed45ace38bba. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XmlnsCache.cs:313

        private void LoadClrnsToAssemblyNameMappingCache(Assembly[] asmList)
        {
            List<ClrNamespaceAssemblyPair> pairList;
            // For each assembly, enmerate all the XmlnsDefinition attributes.
            for(int asmIdx=0; asmIdx<asmList.Length; asmIdx++)
            {
                string assemblyName = asmList[asmIdx].FullName;
                CustomAttributeData[] attributes = GetAttributes(asmList[asmIdx],
                                                "System.Windows.Markup.XmlnsDefinitionAttribute");
                for(int attrIdx=0; attrIdx<attributes.Length; attrIdx++)
                {
                    string xmlns = null;
                    string clrns = null;

                    GetNamespacesFromDefinitionAttr(attributes[attrIdx], out xmlns, out clrns);

                    if (string.IsNullOrEmpty(xmlns) || string.IsNullOrEmpty(clrns) )
                    {
                        throw new ArgumentException(SR.Format(SR.ParserAttributeArgsLow, "XmlnsDefinitionAttribute"));
                    }

                    if (!_cacheTable.Contains(xmlns))
                    {
                        _cacheTable[xmlns] = new List<ClrNamespaceAssemblyPair>();
                    }
                    pairList = (List<ClrNamespaceAssemblyPair>)_cacheTable[xmlns];
                    pairList.Add(new ClrNamespaceAssemblyPair(clrns, assemblyName));
                }
            }
        }

#else
        // Get a list of (clrNs, asmName) pairs for a given XmlNs from a
        // given list of Assemblies.  This returns a list that the caller
        // Will add to the _cacheTable.   At runtime the needed assemblies
        // appear with various XmlNs namespaces one at a time as we read the BAML
        private List<ClrNamespaceAssemblyPair> GetClrnsToAssemblyNameMappingList(

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